Chapter 7: Nucleic Acids and Protein Synthesis

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Chapter 7 Nucleic Acids and Protein Synthesis

• Section 1 DNA

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DNA

• Cells are pre-instructed by a code, or
  programmed, about what to do and how to do it
• A code in living cells must be able to duplicate
  itself quickly and accurately and must also have
  a means of being decoded and put into effect

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The Genetic Code

• Biologists call the program of the cell the
  genetic code
• The word genetic refers to anything that relates
  to heredity
• The genetic code is the way in which cells store
  the program that they seem to pass from one
  generation of an organism to the next generation

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The Genetic Code

• In 1928, the British scientist Frederick Griffith
  was studying the way in which certain types of
  bacteria cause the disease pneumonia
• Griffith had two slightly different strains of
  pneumonia bacteria in his lab
• Both strains grew very well in petri dishes in
  his lab, but only one strain actually caused the
  disease
• The disease-causing strain of bacteria grew into
  smooth colonies on culture plates, whereas the
  harmless strain produced rough colonies
• The differences in appearance made the two
  strains easy to distinguish

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The Genetic Code

• When Griffith injected mice with the
  disease-causing strain of bacteria, the mice got
  pneumonia and died
• When mice were injected with the harmless strain,
  they did not get pneumonia and they did not die
• And when mice were injected with the
  disease-causing strain that had been killed by
  heat, these mice too survived
• By performing this 3rd experiment, Griffith
  proved to himself that the cause of pneumonia was
  not a chemical poison released by the
  disease-causing bacteria

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Transformation

• Next Griffith did an experiment that produced an
  astonishing result
• He injected mice with a mixture of live cells
  from the harmless strain and heat-killed cells
  from the disease-causing strain
• The mice developed pneumonia!

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Transformation

• Somehow Griffiths heat-killed strain had passed
  on its disease-causing ability to the live
  harmless strain
• To confuse matters even more, Griffith recovered
  bacteria from the animals that had developed
  pneumonia
• When these bacteria were grown in petri dishes,
  they formed smooth colonies characteristic of the
  disease-causing strain
• One strain of bacteria had been transformed into
  another
• transformation

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The Transforming Factor

• In 1944, a group of scientists at the Rockefeller
  Institute in NYC led by Oswald Avery, Maclyn
  McCarty, and Colin MacLeod decided to repeat
  Griffiths work and see if they could discover
  which molecules were Griffiths transforming
  factor

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The Transforming Factor

• Avery and his colleagues made an extract from the
  heat-killed bacteria
• When they treated the extract with enzymes that
  destroy lipids, proteins, and carbohydrates, they
  discovered that transformation still occurred
• These molecules were not responsible for the
  transformation
• If they were, transformation would not have
  occurred because the molecules would have been
  destroyed by the enzymes

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The Transforming Factor

• Avery and the other scientists repeated the
  experiment, this time using enzymes that would
  break down RNA (ribonucleic acid)
• Transformation took place again
• But when they performed the experiment again,
  using enzymes that would break down DNA
  (deoxyribonucleic acid), transformation did not
  occur
• DNA was the transforming factor!
• DNA is the nucleic acid that stores and transmits
  the genetic information from one generation of an
  organism to the next
• DNA carries the genetic code

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Bacteriophages

• The work of Avery and his colleagues clearly
  demonstrated the role of DNA in the transfer of
  genetic information
• However, more experiments were needed to solidify
  the findings
• In 1952, Alfred Hershey and Martha Chase did
  experiments with types of bacteria that infect
  viruses
• Bacteriophages
• bacteria eaters
• Composed of a DNA core and a protein coat
• Attach themselves to the surface of a bacterium
  and then inject a material into the bacterium

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Bacteriophages

• Once inside, the injected material begins to
  reproduce, making many copies of the
  bacteriophage
• Because the material injected into the bacterium
  produces new bacteriophages, it must contain the
  genetic code
• Hershey and Chase set out to learn whether the
  protein coat, the DNA, or both was the material
  that entered the bacterium

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Bacteriophages

• From their experiments, it was clear that the
  viruses DNA enters the bacteria
• This was convincing evidence that DNA contains
  the genetic information

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The Structure of DNA

• DNA is a polymer formed from units called
  nucleotides
• Each nucleotide is a molecule made up of three
  basic parts
• A 5-carbon sugar called deoxyribose
• A phosphate group
• A nitrogenous base

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The Structure of DNA

• DNA contains four nitrogenous bases that are
  grouped as either a purine or a pyrimidine
• Purines
• Adenine
• Guanine
• Pyrimidines
• Cytosine
• Thymine

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X-Ray Evidence

• In the early 1950s, Rosalind Franklin turned her
  attention to the DNA molecule
• She purified a large amount of DNA and then
  stretched the DNA fibers in a thin glass tube so
  that most of the strands were parallel
• Then she aimed a narrow x-ray beam on them and
  recorded the pattern on film
• When x-rays pass through matter, they are
  scattered, or diffracted
• Provides important clues to the structure of many
  molecules

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X-Ray Evidence

• Franklin worked hard to prepare better and better
  samples until the x-ray patterns became clear
• The results of her work provided important clues
  about the structure of DNA
• The fibers that make up DNA are twisted, like the
  strands of a rope
• Large groups of molecules in the fiber are spaced
  out at regular intervals along the length of the
  fiber

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Building a Model of DNA

• Two young scientists in England were also trying
  to determine the structure of DNA
• Francis Crick
• James Watson
• Watson and Crick had been trying to solve the
  mystery of DNA structure by building 3D models of
  the atomic groups in DNA
• They twisted and stretched the models in
  different ways to see if any of the structures
  formed made any sense
• No luck

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Building a Model of DNA

• Then, during a visit to London, Watson was able
  to observe Franklins remarkable X-ray pattern of
  DNA
• At once Watson and Crick realized that there was
  something important in that pattern
• Within weeks, Watson and Crick had figured out
  the structure of DNA

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The Double Helix

• Working with these clues, what they needed to do
  was twist their model into a shape that would
  account for Franklins X-ray pattern
• Before long, they developed a shape that seemed
  to make sense
• Helix
• Using Franklins idea that there were probably
  two strands of DNA, Watson and Crick imagined
  that the strands were twisted around each other
• Double helix

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The Double Helix

• The nitrogenous bases on each of the strands of
  DNA are positioned exactly opposite each other
• This positioning allows weak hydrogen bonds to
  form between the nitrogenous bases adenine (A)
  and thymine (T), and between cytosine (C) and
  guanine (G)

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The Double Helix

• Erwin Chargaff, another scientist, provided
  insight to Watson and Cricks work
• Chargaff observes that in any sample of DNA, the
  number of adenine molecules was equal to the
  number of thymine molecules\the same was true for
  the number of cytosine and guanine molecules
• A pairs with T
• C pairs with G
• Base pairing
• the force that holds the two strands of the DNA
  double helix together

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The Double Helix

• In 1953, Watson and Crick submitted their
  findings to a scientific journal
• It as almost immediately accepted by scientists
• The important of this work on DNA was
  acknowledged in 1962 by the awarding of the Nobel
  prize
• Because Rosalind Franklin had died in 1958 and
  Nobel prizes are given only to living scientists,
  the prize was shared by Watson, Crick, and
  Franklins associate, Maurice Wilkins

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The Replication of DNA

• Because each of the two strands of DNA double
  helix has all the information, by the mechanism
  of base pairing, to reconstruct the other half,
  the strands are said to be complementary
• Even in a long and complicated DNA molecule, each
  half can specifically direct the sequence of the
  other half by complementary base pairing
• Each strand of the double helix of DNA serves as
  a template, or pattern, against which a new
  strand is made

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The Replication of DNA

• Before a cell divides, it must duplicate its DNA
• This ensures that each resulting cell will have a
  complete set of DNA molecules
• This copying process is known as replication
• DNA replication, or DNA synthesis, is carried out
  by a series of enzymes
• These enzymes separate, or unzip, the two
  strands of the double helix, insert the
  appropriate bases, and produce covalent
  sugar-phosphate links to extend the growing DNA
  chains

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The Replication of DNA

• The enzymes even proofread the bases that have
  been inserted to ensure that they are paired
  correctly
• DNA replication begins when a molecule of DNA
  unzips
• The unzipping occurs when the hydrogen bonds
  between the base pairs are broken and the two
  strands of the molecule unwind

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The Replication of DNA

• Each of the separated strands serves as a
  template for the attachment of complementary
  bases
• For example, a strand that has the bases
  T-A-C-G-T-T produces a strand with the
  complementary bases A-T-G-C-A-A
• In this way, two DNA molecules identical to each
  other and to the original molecule are made

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Chapter 7 Nucleic Acids and Protein Synthesis

• Section 2 RNA

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RNA

• The double helix explains how DNA can be
  replicated
• However, it does not explain how information is
  contained in the molecule or how that information
  is used
• DNA contains a set of instructions that are coded
  in the order of nucleotides

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RNA

• The first step in decoding that message is to
  copy part of the sequence into RNA (ribonucleic
  acid)
• RNA is the nucleic acid that acts as a messenger
  between DNA and the ribosomes and carries out the
  process by which proteins are made from amino
  acids

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The Structure of RNA

• RNA is made up of nucleotides
• There are three major differences between RNA and
  DNA
• The sugar in RNA is ribose
• RNA is a single strand
• RNA contains the bases adenine, guanine,
  cytosine, and uracil
• A pairs with U
• C pairs with G

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The Structure of RNA

• A cell contains many different forms of RNA
• An RNA molecule is a disposable copy of a segment
  of DNA

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Transcription RNA Synthesis

• In RNA synthesis, the molecule being copied is
  just one of the two strands of a DNA molecule
• Transcription is the process by which a molecule
  of DNA is copied into a complementary strand of
  RNA
• Transferring information from DNA to RNA

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Transcription RNA Synthesis

• Why do we need to do this?
• DNA does not leave the nucleus so we need a
  messenger to bring the genetic information from
  the DNA in the nucleus out to the ribosomes in
  the cytoplasm
• Messenger RNA (mRNA)

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Transcription RNA Synthesis

• During transcription, the enzyme RNA polymerase
  attaches to special places on the DNA molecule,
  separates the two strands of the double helix,
  and makes a mRNA strand
• The mRNA strand is complementary to one of the
  DNA strands
• The base pairing mechanism ensures that the mRNA
  will be a complementary copy of the DNA strand
  that serves as its template

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Transcription RNA Synthesis

• Special sequences in DNA serve as start signals
  and are recognized by RNA polymerase
• Other areas on the DNA molecule are recognized as
  termination sites where RNA polymerase releases
  the newly synthesized mRNA molecules

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Chapter 7 Nucleic Acids and Protein Synthesis

• Section 3 Protein Synthesis

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Protein Synthesis

• The nitrogenous bases in DNA contain information
  that directs protein synthesis
• Because most enzymes are proteins, proteins
  control biochemical pathways within the cell
• Not only do proteins direct the synthesis of
  lipids, carbohydrates, and nucleotides, but they
  are also responsible for cell structure and cell
  movement

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The Nature of the Genetic Code

• Proteins are made by stringing amino acids
  together to form long chains called polypeptides
• Each polypeptide contains a combination of any or
  all of the 20 different amino acids
• DNA and RNA each contain different nitrogenous
  bases

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The Nature of the Genetic Code

• In order to code for the 20 different amino
  acids, more than one nucleotide must make up the
  code word for each amino acid
• The code words of the DNA nucleotides are copied
  onto a strand of messenger RNA
• Each combination of three nucleotides on the
  messenger RNA is called a codon
• Each codon specifies a particular amino acid that
  is to be placed in the polypeptide chain

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The Nature of the Genetic Code

• There is one codon, AUG, that can either specify
  the amino acid methionine or serve as a started
  for the synthesis of a protein
• Start codon
• There are also three stop codons
• These codons act like the period at the end of a
  sentence
• Signify the end of a polypeptide

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Translation

• The decoding of mRNA into a protein is known as
  translation
• The mRNA does not produce a polypeptide by itself
• Instead, there is a mechanism that involves the
  two other main types of RNA and the ribosome
• Transfer RNA (tRNA)
• Carries amino acids to the ribosomes
• Ribosomal RNA (rRNA)
• Makes up the major part of the ribosomes

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The Role of Transfer RNA

• In order to translate the information from a
  single codon of mRNA, such as AUG, we would have
  to find out which amino acid is coded for by AUG
• The codon AUG codes for the amino acid methionine
• Methionine is then brought to the polypeptide
  chain by tRNA

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The Role of Transfer RNA

• There are three exposed bases on each tRNA
  molecule
• These nucleotides will base pair with a codon on
  mRNA
• Because these three nucleotides on tRNA are
  complementary to the three nucleotides on mRNA,
  the three tRNA nucleotides are called the
  anticodon

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The Role of Transfer RNA

• Attached to each tRNA molecule is the amino acid
  specified by the codon to which it base pairs
• By matching the tRNA anticodon to the mRNA codon,
  the correct amino acid is put into place
• Each tRNA acts like a tiny beacon for its
  specific amino acid

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The Role of the Ribosome

• The process of protein synthesis takes place in
  the ribosomes
• Ribosomes are made up of two subunits
• Proteins
• rRNA
• The first part of protein synthesis occurs when
  the two subunits of the ribosome bind to a
  molecule of mRNA
• The AUG binds to the first anticodon of tRNA,
  signaling the beginning of a polypeptide

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The Role of the Ribosome

• Soon the anticodon of another tRNA binds to the
  next mRNA codon
• This second tRNA carries the second amino acid
  that will be placed into the chain of the
  polypeptide
• As each anticodon and codon bind together, a
  peptide bond forms between the two amino acids

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The Role of the Ribosome

• The polypeptide chain continues to grow until the
  ribosome reaches a stop codon on the mRNA
• When the stop codon reaches the ribosome, the
  ribsome releases the newly formed polypeptide,
  completing the process of translation